Mongoose Stat 29" disc, $219

http://www.walmart.com/ip/29-Mongoose-Stat-Men-s-Mountain-Bike/21191093

Mongoose Stat
-Aluminum frame, size 18"
-29" wheels, fast rolling, comfortable, good selection of tires
-disc brakes.  More consistent in wet weather.  The brakes don't wear out your rims, like rim brakes do.  Brake pads and rims last longer with disc brakes.

Is the stat a decent bike?  It sells for $220, and has been on sale for as low as $160.

The aluminum frame is pretty good quality, and has some nice details, especially for the price.  It has a common 1 1/8" headtube, 68mm euro bb shell, 27.2mm seatpost.  It comes in one size, 18".  Good for 5'8"-5'11" riders.

The fork is a very heavy, cheap, steel suspension forks.  This is one of the worst parts of the bike.  The good news is that it is 1 1/8" threadless steerer tube, the most common size.

A chromoly steel rigid fork would be a good replacement.  This is simpler, lighter, no maintenance, easier to fit fenders.  You don't want shocks on a commuter bike.
This $50 Nashbar fork is a good choice.  Axle to crown: 453mm.

The Stat uses 1 1/8" stem, with 25.4mm handlebar.  These are the most common sizes.  Easy to swap out different styles of handlebars, or different length stems.
Stems are <$20
Bars are $15-35 depending on style.

The wheels use Quando aluminum quick release hubs, F&R.  It uses a freewheel in the rear.  Double wall 29" aluminum rims.  These are pretty nice wheels.
The tires are cheap, knobby tires.  They roll slow, and get flats easily.  Faster rolling, more flat resistant commuter tires are the best change you can make to this bike.  Continental, Michelin, Vittoria, Rubena make nice 29x2.0" fatty city tires for around $20-30 per wheel

29" rims and 700c rims are the same diameter, and their tires are usually interchangeable.  Both 29" and 700c rims have the same 622mm ERTO diameter.  29" rims tend to be wider to work with fatter tires.  700c tend to be narrower to work with skinnier tires.
29" tires are measured width in inches, for example 2.0".
700c is by "c", bacially mm's.  23c= 23mm very skinny.  You can fit fatter 35c and up tires on 29" rims.

These Schwalbe 700 X 40c tires are a bit narrower and faster rolling.  40mm is about 1.6" wide, so they still have some cush.
$25 for the pair is a great deal.

Fenders are another thing to think about when choosing tires.  There are not that many fenders available for fatty 29X2+" tires.  The Planet Bike Cascadia fenders are an excellent choice, but pricey at $45.

More common sizes are 700X45mm fenders, $23.  These only fit smaller width 700c tires, 44c or lower.
Going this route, you are under $50 for good tires and fenders.

The Promax cable disc brake calipers are not terrible.  One bad thing is their brake pads are not commonly stocked at bike shops.

Replacement Cheap brake pads on ebay

Better brake calipers, that take commonly stocked brake pads would be a good upgrade for this bike.
Avid BB-5., $33  These are good brakes and brake pads are readily available for them.  The are better constructed, and easier to adjust.  The promax's are not bad, though.

The brake levers have a plastic body, with an aluminum lever blade.  They work, but they are flexier than levers with an aluminum lever body.  The cables and housing on the bike are poor quality.  Better levers and cables would really improve this bike.
Shimano Acera levers, including cables & housing, $14

The 3 X 7 speed SRAM grip shifters are good.  The shift cables & housing on the bike are poor quality.  Shimano, SRAM, Jagwire make good shift cables & housings.

Shimano Tourney derailleur.
7 speed freewheel & chain.  Drivetrains usually last about 3000 miles.  The replacement parts for this bike are very inexpensive.  The chain and freewheel on the bike are cheap chinese.
The 7 speed freewheel costs $12 to replace with good quality shimano one.
Better quality SRAM chain is $8.
This bike has one of the most affordable drivetrains to replace.

The cranks are chinese, aluminum arms, with stamped steel rings.  They are not light, but they are durable.  The bottom bracket is euro threaded adjustable loose ball.  Replacement shimano cranks are around $40, and bottom brackets around $20.  Though I would expect the cranks to last awhile.

The pedals are pretty nice aluminum BMX pedals.  Most walmart bikes have janky plastic pedals, these ones are good.

The seat and post.  The quality of the seat is not terrible, but it is heavy and has a really tacky cover.  It is one of the cheapest looking things on the bike.

The seatpost is a cheap "seat guts" style post.  These are heavy and prone to creaking and slipping.

 A 2 bolt Kalloy 27.2mm seatpost would be a good upgrade for <$20.

Kalloy 2 bolt aluminum seatpost $14

Singleworks city limit, leather, $20

Overall, the bike is not a bad choice for a commuter.  The worst parts are the tires and fork.  The brakes would be better with quality levers and cables.  Decent seat and post would save weight, look better.
Tires= $25
Fenders= $22
-------------------
Fork= $50
Levers= $13
Seat & post= $34
$144 total.